The present invention relates to a signal processing apparatus, a semiconductor chip, a signal processing system, and a method of processing a signal.
A conventional signal processing apparatus is disposed on an input path for recording an audio signal, so that the conventional signal processing apparatus adjusts audio quality of the audio signal thus input. For example, when a voice is captured and recorded through a microphone, the conventional signal processing apparatus is provided for reducing a wind noise generated by wind blowing into the microphone.
FIG. 4 is a graph showing a frequency characteristic of a high pass filter (an HPF) of the conventional signal processing apparatus. In general, a wind noise contains a large noise component in a low frequency range. Accordingly, the conventional signal processing apparatus is provided with the HPF for eliminating the large noise component in the low frequency range. However, when the audio signal is processed only with the HPF, any component in the low frequency range is reduced even when there is little wind noise. Accordingly, a necessary audio component such as an environmental sound may be reduced as well, thereby making it difficult to obtain high audio quality.
FIG. 5 is a block diagram showing a configuration of the conventional signal processing apparatus. As shown in FIG. 5, the conventional signal processing apparatus includes a wind noise filter having a high pass filter 100 (an HPF) and a low pass filter 102 (an LPF). An audio signal is input into the HPF 100 and the LPF 102. The HPF 100 is provided for extracting a high frequency component including a human voice from the audio signal thus input. The LPF 102 is provided for extracting a low frequency component from the audio signal thus input. A signal output from the LPF 102 is input to a volume adjusting circuit 104 (VOL) as an attenuator with a constant gain, so that the VOL 104 attenuates and outputs the low frequency component. An accumulator 106 is provided for accumulating (combining) a signal output from the VOL 104 with a signal output from the HPF 100.
In the conventional signal processing apparatus having the wind noise filter as shown in FIG. 5, the low frequency component, in which a wind noise causes a problem most, is reduced. With the configuration shown in FIG. 5, the low frequency component is not completely eliminated, but rather maintained at a specific level while a wind noise is reduced.
FIG. 6 is a graph showing a frequency characteristic of the wind noise filter having the high pass filter 100 (the HPF) and the low pass filter 102 (the LPF) of the conventional signal processing apparatus. As shown in FIG. 6, when the conventional signal processing apparatus is provided with the wind noise filter having the HPF 100 and the LPF 102, it is possible to obtain better audio quality even when a wind noise is small as compared with the conventional signal processing apparatus provided with only the HPF.
Patent Reference: Japanese Patent Publication No. 06-319193
Patent Reference has disclosed another conventional signal processing apparatus. The conventional signal processing apparatus disclosed in Patent Reference includes a unit for determining a gain to attenuate the low frequency component according to an audio intensity difference between the high frequency component passing through the HPF and the low frequency component passing through the LPF.
FIG. 7 is a block diagram showing a configuration of the conventional signal processing apparatus disclosed in Patent Reference. Components shown in FIG. 7 and similar to those in FIG. 5 are designated with the same reference numerals, and explanations thereof are omitted.
In the conventional signal processing apparatus shown in FIG. 7, a signal output from the HPF 100 is input to a high frequency level detecting unit 110, so that the high frequency level detecting unit 110 detects a level of the signal output from the HPF 100. A signal output from the LPF 102 is input to a low frequency level detecting unit 112, so that the low frequency level detecting unit 112 detects a level of the signal output from the LPF 102.
Further, the signal output from the LPF 102 is input to a volume adjusting circuit 116 (VOL) as an attenuator with a variable gain, so that the VOL 116 attenuates and outputs the low frequency component. The accumulator 106 is provided for accumulating (combining) a signal output from the VOL 116 with the signal output from the HPF 100.
In the conventional signal processing apparatus shown in FIG. 7, a level comparing unit 114 is provided for determining the variable gain of the VOL 116. More specifically, the level comparing unit 114 compares the detection result input from the high frequency level detecting unit 110 with the detection result input from the low frequency level detecting unit 112.
When the low frequency level is not so high as opposed to the high frequency level, indicating that a wind noise is not so high, the level comparing unit 114 sets the variable gain of the VOL 116 so that the low frequency level is not reduced excessively. When the low frequency level is significantly high as compared with the high frequency level, indicating that a wind noise is very high, the level comparing unit 114 sets the variable gain of the VOL 116 so that the low frequency level is reduced accordingly.
FIG. 8 is a graph showing a frequency characteristic of the wind noise filter having the high pass filter 100 (the HPF) and the low pass filter 102 (the LPF) of the conventional signal processing apparatus disclosed in Patent Reference. As shown in FIG. 8, when the conventional signal processing apparatus is provided with the unit for determining the gain to attenuate the low frequency component, it is possible to obtain the frequency characteristic in which only the low frequency level is variable. More specifically, when a wind noise is not so high, the low frequency level is not reduced excessively. When the wind noise is very high, the low frequency level is reduced accordingly. As a result, it is possible to obtain better audio quality even when the wind noise is small as compared with the conventional signal processing apparatus shown in FIG. 5.
In the conventional signal processing apparatus shown in FIG. 7, an auto level controller is normally provided as a later stage unit for amplifying a signal output from the accumulator 106. However, the auto level controller is designed to amplify the signal such that a whole wave shape is maintained at a constant level. Accordingly, it is difficult to adjust a volume according to a human voice, which is a main subject of listening, thereby making it difficult to obtain a resultant audio signal with high quality for being easily discernable.
In view of the problems described above, an object of the present invention is to provide a signal processing apparatus capable of solving the problems of the conventional signal processing apparatus. A further object of the present invention is to provide a semiconductor chip, a signal processing system, and a method of processing a signal. In the present invention, it is possible to obtain an audio signal with high quality for being easily discernable as well as reduce a wind noise.
Further objects and advantages of the invention will be apparent from the following description of the invention.